8 Packet Shaping and QoS (1) Each TCP connection tries to use as much bandwidth as it can get, and backs off when packets are lost or delayed. When a data link becomes heavily saturated by multiple competing applications: ACKs can be delayed limiting throughput Efficiency drops as data is retransmitted Connections back off to avoid congestion The purpose of Packet Shaping and QoS is to adapt network behavior to incorporate your traffic priorities and increase TCP/IP efficiency over the slow link or last mile.

9 Packet Shaping and QoS (2) Three basic techniques: Queuing (buffering and waiting) to limit send rate. Since TCP is adaptive, it will self adjust to the impaired data link. Examples: throttled, LinkSys WRT-54, Xincom, WonderShaper (Linux), pf and ALTQ (FreeBSD) Expedite higher priority traffic by sending out of order or prioritizing among queues. Handle ACK prioritization as a special case. Example: throttled, pf and ALTQ Use TCP s built-in flow control mechanism to modulate the send rate for maximum efficiency. Examples: Packeteer, IPNetRouterX (IPNetSentryX) No open source implementation at this time

10 Packet Shaping Techniques (1) Queue Queuing (buffering and waiting) Easiest to implement, can be applied to any protocol Only rate limits send traffic (directly) Limited number of queues (compared to TCP itself) Shaping amounts to delaying, re-ordering, and dropping packets Important to prioritize empty TCP ACKs. Can be very effective depending on the problem, but not as powerful.

11 Packet Shaping Techniques (2) Offered window (advertised by receiver) Sent and ACKed Sent not ACKed (in-flight) Can send ASAP (useable window) Cant send until window moves (ACK received) TCP Rate Limiting Adjust receive window on the fly to control when and how much the window moves. Can rate limit send and receive traffic. Avoids congestion for best use of available bandwidth.

19 Server Pacing Consider a web server behind a T1 line (1.544 Mbps) that recieves two requests for a large web page. On Mac OS X, the TCP send window defaults to 32K. The sender ramps up to 32K in flight for each request. At T1 speeds it takes over 300ms to send the 64K in flight, any other requests will need to wait behind this backlog. With rate limiting, the total send window is limited to 1/10 the specified rate limit. If we use 1.5 Mbps, the total send window will be 20K or about 100ms. The server still responds immediately and pages arrive just as quickly. We just parcel out the data more slowly to avoid creating a backlog.

20 Reserve Bandwidth for VOIP Does your VOIP phone sound worse than your cell phone? If downstream is saturated by other file transfers, need to slow these down. Simple queue and drop won t work since the traffic is inbound and we need to slow the individual data flows, not outbound packets

21 Good Fences Make Good Neighbors Want to share your WiFi but limit how much bandwidth it actually gets? Do your kids sometimes hog your connection?

22 Back to Partitioning Router The intent of partioning is to keep unwanted traffic out of the way. Another way of doing this is to relocate resources to where they are needed and police unwanted traffic.

24 Content Caching Squid is the most popular proxy server in use today and even has Macintosh friendly front ends such as SquidMan or Maxum iassist to keep installation and configuration simple. Typically, caching can save 20-30% of the bandwidth and improve browsing speeds, especially over slow or congested network connections. UNIX bind supports local caching DNS server. See bind9arm for details or use a GUI front end.

25 Dead Gateway Detection Similar to Windows behavior When a connection attempt is retransmitted for the 3rd time, select an alternate gateway by creating a new host route on the fly. If more than half of active connections have failed over to alternate gateway, make it the default gateway by selecting a new location. Transparent fail over before connection attempt times out. Cable NAT Gateway 1 DSL NAT Gateway 2 Host

26 Tuning for Performance Why is it necessary to tune TCP/IP? TCP/IP is not controlled by any manufacturer and is designed to work on almost any kind of underlying network with widely varying characteristics. To meet this challenge, the protocol designers made TCP adaptive. TCP is self monitoring and optimizes its own behavior to match the network environment. Adaptation takes time and the default settings cannot optimize for every possible kind of network. There are many independent implementations, the protocols continue to evolve in response to practical experience, some implementations have peculiar compatibility constraints.

27 Typical Tuning Problems Asymmetric connections that are much faster downstream than upstream. High performance connections with relatively long latency (measured in bytes) such as fiber optic, satellite, or PPP via cellular network. PPPoE implementations that restrict the network MTU without support for Path MTU Discovery or fragmentation.

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